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1. Field of the Invention
The present invention relates to the placement of pipelines in a marine environment. More particularly, the present invention relates to an improved method and apparatus for laying a pipeline in a marine environment, wherein a specially configured pipeline laying ship provides a hull with a deck featuring a pipeline support truss that has a lower fixed section, an upper pivoting section and a launch section. A pipeline fabricating xe2x80x9cfiring linexe2x80x9d fabricates a pipeline that is routed to the support truss as it is constructed. The launch section is provided for directing the pipeline from the support truss to tensioning and straightening stations and then preferably through a moon pool in the hull to the seabed.
2. General Background of the Invention
For many years, pipelines have been fabricated at sea and lowered to the seabed at a desired location. For example, early patents to Tesson include U.S. Pat. Nos. 3,237,438 and 3,372,461 that are directed to a pipeline laying barge and method of operation. In the Tesson patents, the joints of pipe are welded on deck and then spooled or wound upon a reel. At a desired location, the pipeline is then placed on the seabed by unwinding the pipe from the reel and straightening it before it is lowered to the seabed.
A tilting tower has also been used to lower a pipeline to the seabed as the pipeline is being unwound from a reel. An example of such a titling tower and reel pipeline laying arrangement can be seen in the Uveda U.S. Pat. No. 4,345,855 as an example. A similar arrangement is seen in the Springett U.S. Pat. No. 4,340,322. The Uveda U.S. Pat. No. 4,345,855 and the Springett U.S. Pat. No. 4,340,322 are hereby incorporated herein by reference. Another method of laying a pipeline is through the use of a stinger. A stinger is simply an elongated boom structure that extends from usually one end of a hull or barge. The stinger supports the pipeline as it is being lowered to the seabed. Examples of the use of stingers can be seen us U.S. Pat. Nos. RE28,922, RE29,591 and 5,533,834.
Reel type pipeline laying systems are typically employed in deep water situations. While it is known to weld the pipeline joints on the deck of a vessel and wind them upon a reel (for example, see the above discussed Tesson patents), it is also known to weld the pipeline on shore and then wind it upon the reel before the ship leaves port.
The following table provides an exemplary listing of patents that are known to applicant and that relate to pipeline laying vessels and methods of operation:
The present invention provides an improved pipeline laying vessel and its method of operation. The method employs a dynamically positioned barge or self-propelled dynamically positioned reel ship. The ship has a hull with a deck area that enables pipe joints to be welded together to form a pipeline. Joints of pipe are placed in a storage area on the deck of a vessel in suitable pipe racks. Welding stations near this storage area are arranged to receive multiple joints of pipe that have been internally cleaned and prepared for welding. Part of this preparation can include, for example, end bevels that are applied to each pipe joint.
As pipeline fabrication proceeds, pipe joints are moved from the rack storage areas to the pipeline fabrication area, also referred to herein as the xe2x80x9cfiring line.xe2x80x9d
This fabrication area or xe2x80x9cfiring linexe2x80x9d can comprise essentially a set of rollers supporting the pipeline along the preferably horizontal center line of the fabrication area. Welding equipment (manual or automatic) can be provided together with known weld preparation tools.
The incoming pipe joints are aligned with the pipeline being fabricated. The welded area between the pipe joints and the joints previously welded together can be adjusted as required. The pipe joints are progressively welded together to form a pipeline at multiple welding stations. The welds are tested and field joints are then coated at another work station located in the pipeline fabrication area.
When one or more joints of pipe have been welded together end-to-end on the firing line, the assembled pipeline is routed through a pipe bender to a support truss. The pipeline moves ahead in the pipeline fabrication area by the same distance, e.g. 40 foot increments.
Once bent to a desired curvature by the pipe bender, the pipeline tracks upon a lower section of the truss support. Tension can be adjusted depending on the various pipe laying parameters such as pipeline characteristics (e.g. diameter, wall thickness, coating, alloy, wet or dry lay)and water depth during lay operations. The support truss has upper and lower sections. The lower section is a base section with feet that rest upon the ship hull deck, preferably in front of the moon pool. The pipeline being welded is fed to the lower section as the pipeline is welded, joints of pipe being added one at a time. The pipeline being welded has a generally horizontal, longitudinally extended axis. The lower section of the support truss preferably tracks a plane that is occupied by the axis of the pipeline. A curved track or ramp is provided on the lower section of the truss. After leaving the pipe bender, the bent pipeline is supported by the curved ramp on the lower truss section. The curved ramp on the lower truss section terminates at a pivotal connection located at the front, upper corner of the lower truss section.
An upper truss section is pivotally attached to the lower truss section at a rotary bearing. The upper truss section has a curved ramp that is of a generally semicircular shape. The ramp begins next to the rotary bearing. The end of the curved ramp of the lower truss section is positioned just below the rotary bearing. The rotary bearing has a central opening that enables the pipeline to travel from the curved ramp of the lower truss section, through the central opening of the rotary bearing and then to the curved ramp of the upper truss section.
The curved ramp of the upper truss section begins at the front of the upper truss section and ends near the rear of the upper truss section that connects to the launch section. The launch section of the truss support is pivotally attached to the upper truss section so that the launch section can be inclined a desired orientation (eg. vertical or up to 35 degrees from vertical).
The proper tilting of the launch section and the coordination of the tension in the pipeline being launched with the ship as it moves along the pipeline route ensure that the stresses in the pipeline are maintained within predetermined, predefined acceptable limits.
On the launch section, an area in between the tensioner and the clamping mechanism or xe2x80x9chold offxe2x80x9d clamp can be a work station that can be used to support the pipeline and install corrosion anodes and insulation coating when required on the pipeline joints that are welded in the pipeline fabrication area or xe2x80x9cfiring line,xe2x80x9d or in the tower itself. The work station can be used to initiate the pipeline, lay down, and install testing devices at the beginning and the end of the pipeline lay operations.